Mucosal Relief and Lining
The inner surface of the esophagus is gathered into 7–10 longitudinal folds formed by the mucosa (tunica mucosa) and submucosa. These folds flatten during the passage of food to expand the lumen.
The mucosa comprises three layers:
- Epithelium: Non-keratinized stratified squamous epithelium, similar to the oral mucosa. In older individuals, it may undergo partial keratinization.
- Lamina propria: Composed of loose connective tissue. It contains cardiac glands (simple branched tubular glands) that secrete a protein-mucus fluid and are restricted to two zones: the level of the cricoid cartilage and the gastroesophageal junction. They contain enteroendocrine cells that produce serotonin. This zone is a frequent site for pathologies (cysts, diverticula, ulcers, tumors).
- Muscularis mucosae: Formed by smooth myocytes. In the upper third of the organ, it consists of isolated bundles that merge further down into a continuous layer.
Submucosa and Glandular Apparatus
The submucosa (tela submucosa) is well-developed and consists of loose connective tissue. Unlike the mucosa, it houses the esophageal proper glands (glandulae esophageae propriae).
- Localization: Throughout the entire length of the esophagus.
- Morphology: Complex branched tubuloacinar glands.
- Function: Secrete mucus to facilitate the sliding of the food bolus.
The excretory ducts of these glands pierce the mucosa and open onto the epithelial surface. The submucosa also contains large vascular and nerve plexuses, as well as lymphoid aggregates.
Muscular and Outer Layers
The muscular layer (tunica muscularis) consists of two functional layers:
- Inner circular layer (forms thickenings acting as sphincters at the beginning and end of the organ).
- Outer longitudinal layer.
Notably, in histological preparations of a canine esophagus, these layers are inverted. Intramural ganglia lie between the muscle layers, providing autonomic innervation.
The key feature of this layer is the transition of muscle tissue types. In the upper third, it is striated (skeletal) muscle; in the middle third, a mixture of skeletal and smooth muscle; and in the lower third, exclusively smooth muscle.
The outer layer over most of its extent (neck and thorax) is an adventitia composed of connective tissue rich in blood vessels. In the short abdominal segment, the esophagus is covered by a serosa (connective tissue covered by mesothelium).
Embryological Development (Histogenesis)
The esophageal epithelium has a mixed ecto-endodermal origin—it develops from the prechordal plate (oropharyngeal membrane), which is typical for the foregut. The remaining wall layers differentiate from the mesenchyme.
During ontogenesis, the epithelium undergoes a complex sequence of transformations:
- Simple columnar.
- Stratified columnar.
- Pseudostratified ciliated.
- Non-keratinized stratified squamous (final adult form).